Porous Polymer Materials for CO2 Capture and Electrocatalytic Reduction

Efficient capture of CO2 and its conversion into other high value-added compounds by electrochemical methods is an effective way to reduce excess CO2 in the atmosphere. Porous polymeric materials hold great promise for selective adsorption and electrocatalytic reduction of CO2 due to their high specific surface area, tunable porosity, structural diversity, and chemical stability. Here, we review recent research advances in this field, including design of porous organic polymers (POPs), porous coordination polymers (PCPs), covalent organic frameworks (COFs), and functional nitrogen-containing polymers for capture and electrocatalytic reduction of CO2. In addition, key issues and prospects for the optimal design of porous polymers for future development are elucidated. This review is expected to shed new light on the development of advanced porous polymer electrocatalysts for efficient CO2 reduction.


Introduction
The usage of fossil energy has enabled human society to prosper, but it has also led to serious environmental problems [1]. Since the burning of coal, petroleum, and natural gas, the CO 2 fixed by plants from nature through millions of years of photosynthesis has been released by humans in just a few hundred years [2]. Moreover, the greenhouse effect caused by excessive CO 2 emissions contributes to the climate crisis, sea level rise, and ocean acidity increase [3][4][5]. Up to now, "zero carbon" or "carbon neutral" climate goals have been set up in more than 130 countries or regions [6]. In order to achieve this goal, research work focuses on finding clean energy alternatives to fossil fuels to reduce CO 2 emissions, and developing technologies such as CO 2 capture, storage, utilization, and chemical conversion to reduce the concentration of CO 2 in the air [7][8][9].
The International Energy Agency predicted that CO 2 capture and storage (CCS) can reduce CO 2 emissions by about 14% annually, which is an important green "bridging" strategy in greenhouse gas reduction [10]. CO 2 capture primarily includes pre-combustion, oxyfuel combustion, and post-combustion. CO 2 sequestration is the last link of the CCS chain, which focuses on marine and geological sequestration and mineral carbonation [11]. In addition, the research focus of CO 2 capture lies on post-combustion technology, which can be applied to the capture of low-pressure CO 2 in flue gases [12]. In industry, CO 2 in mixed exhaust gases can be effectively captured by amine solutions such as monoethanolamine (MEA) and diethanolamine (DEA). However, the high degree of toxicity, corrosion risk, as reviews have covered the studies of porous polymers in the field of CO 2 capture and conversion [31,32], but a thorough overview of the structure-activity relationship, especially the effect of porosity on the capture capacity and catalytic activity of CO 2 is still lacking. In this review paper, the porosity regulation in porous polymers and the influence of pore sizes on CO 2 capture and electrocatalytic reduction are comprehensively summarized. Special attention will be paid to the interaction between pore structure with CO 2 and reaction intermediates. Porous polymers as CO 2 adsorbents and catalysts for CO 2 electrocatalytic reduction will be discussed in this paper. Moreover, the existing limitations, the challenges in porous polymer design, and the future research direction have also been prospected.

Porous Polymers for CO 2 Capture and Electrocatalytic Reduction
Porous polymer materials used for CO 2 capture and electrocatalytic reduction are mainly divided into four categories, namely, porous organic polymers (POPs), porous coordination polymers (PCPs), covalent organic frameworks (COFs), and nitrogen-containing polymers (N-polymers). Such porous polymeric materials can be prepared through a variety of synthetic methods, including solvothermal, ionothermal, microwave, mechanochemical, and interfacial synthesis methods. Among them, the solvothermal method is the most common preparation method [33]. The advantage of the solvent thermal method is that it can improve the synthesis efficiency by changing the solvent, but the biggest problem is that the solvent used is mostly organic solvent, which will cause environmental pollution when used in large quantities, and the reaction conditions are harsh, so it is not suitable for large-scale production. The ionothermal method is mainly used for the synthesis of covalent triazine frameworks (CTFs), and the reaction conditions required are more severe than those of the solvent thermal method, so the ionothermal method is not suitable for industrial applications [34]. The microwave method can effectively reduce the reaction time and maintain the porosity by microwave, which is a promising synthesis method [35]. The mechanochemical method does not require solvents and therefore avoids environmental pollution and is a good green synthesis method because of the simplicity of the process [36].
The CO 2 Reduction Reaction (CO 2 RR) generally takes three steps: CO 2 adsorption, electrocatalytic reduction, and desorption. To be specific, gaseous CO 2 molecules are first dissolved in the solvent and form a CO 2 -saturated electrolyte. Then, the dissolved CO 2 in the solvent reacts on the electrode surface. Finally, the reaction products are desorbed from the electrode surface to complete the whole reaction process. Because of the large number of products and complex intermediates in multi-step reactions of CO 2 RR, definite reaction mechanisms remain unclear. Currently reported CO 2 RRs occurring in aqueous solutions are summarized in Table 1. From the reaction equations in the table, it can be seen that CO 2 can undergo different degrees of electrochemical reduction reactions by transferring different numbers of electrons, and the products include HCOOH, CH 3 OH, CO, CH 4 , CH 3 COOH, C 2 H 4 , CH 3 CH 2 OH, C 2 H 6 , etc. Additionally, the reduction of CO 2 is accompanied by HER on the cathode.

Porous Organic Polymers (POPs)
Porous organic polymers (POPs) are 2D or 3D network structures expanded by organic groups or fragments generated through chemical polymerization [37,38]. With a large specific surface area, variable porosity, excellent thermal stability, as well as controllable functionality, POPs are widely applied in gas adsorption, heterogeneous catalysis, energy storage, and other fields [36,[39][40][41][42]. A large specific surface area and abundant porosity lead to high CO 2 absorption capacity, bringing out the importance of pore regulation in the design of CO 2 capture materials ( Table 2).
In recent years, many studies have focused on POPs serving as hosts or heteroatom catalysts to capture and convert CO 2 [31,[43][44][45]. Han et al. [46] obtained 1,3-dialkyne linked porous polymer LKK-CMP-1 through 1,3,6,8-Tetraethynylpyrene homopolymerization for CO 2 capture. The N 2 adsorption/desorption isotherm for LKK-CMP-1 is a typical I-type, indicating a microporous structure (Figure 1a). The pore size of LKK-CMP-1 is mainly distributed around 0.59 nm (Figure 1b), close to the dynamic diameter of CO 2 (0.33 nm). The CO 2 isosteric heat (Q st ) for LKK-CMP-1 was 35.0 kJ mol −1 (Figure 1c), which proves CO 2 molecules have a strong interaction with LKK-CMP-1 pore walls. The CO 2 adsorption capacity can reach 9.78 wt% at 273 K (1 bar). Song et al. [47] tuned the size of micropores through side-chain engineering. Crosslinkable dianhydride monomers PEPHADA and PEQDA were utilized to construct microporous networks ( Figure 1d). The crosslinkable alkyne group on the PDQDA side branch is closer to the molecular skeleton, which adjusts the pore size of porous polymers to 0.57 nm (Figure 1e). Therefore, the pore size of crosslinked polyimide can be precisely regulated by adjusting the position of the crosslinkable group in polyimide. The 6FA-PE-CL with well-developed micropores shows a higher surface area and stronger CO 2 uptake ability. At 273 K and 1 bar, the CO 2 uptake ability of 6FA-PE-CL is 8.9 wt% (Figure 1f). 6FA-PE-CL exhibits a significant Q st value downward trend compared to 6FA-PEPH-CL with the increase of CO 2 adsorption, which indicates that there is a stronger interaction between ultramicropores (0.56 nm) and CO 2 molecules.
POP is an ideal porous structure, which can be applied for the electrocatalytic reduction of CO 2 through functional design. Dai et al. [48] reported a porous triazine linking framework TTF-1 with a pyridine linker as the reactive site ( Figure 2a). The existence of a well-developed porous structure enables a large contact area between electrodes and electrolytes, and facilitates the charge and mass transfer ( Figure 2b). Amorphous porous polymer POP often shows a broad pore size distribution, which is attributed to the interparticle void. In particular, a large number of micropores promote the adsorption of CO 2 . The permanent CO 2 uptake of TTF-1 is up to 64.4 cm 3 g −1 (298 K, 1 bar). TTF-1 also shows excellent electrocatalytic activity for CO 2 reduction, with a Faraday efficiency of about 82%.
The size of the pore can affect the charge density of the polymer and the confinement of the pore to the active site. Tang et al. [49] synthesized a series of porous polymers, POP-Py (n) (n means the quantity of benzene rings), through the reaction of benzyl bromide and pyridine. POP-Py (n) with pore sizes from micropore to mesopore were obtained by changing the amount of benzene rings (Figure 3a). The highly efficient electrocatalyst was obtained by encapsulating cobalt meso-tetra(4-carboxyphenyl)porphyrin (CoTCPP) (Figure 3b) into POP-Py (n) by the ion exchange method, with single-molecule dispersion. POP-Py (0)/CoTCPP showed the highest FE co of 83% at −0.6 V vs. RHE, which is twice as high as the activity of the pure CoTCPP molecular electrocatalytic conversion of CO 2 to CO (Figure 3c). The high activity is caused by the monomolecular dispersion of CoTCPP in the pores of POP-Py (0), which promotes the electrochemical accessibility of active sites. Moreover, the positively charged substituted pyridine ring in POP-Py can stabilize the key reaction intermediate *COOH to accelerate the conversion of CO 2 . When a neutral benzyl ring is inserted between two pyridine rings, the positive charge on the pyridine ring will be delocalized through the conjugation effect. Hence, the interaction between POP-Py and key reaction intermediate *COOH becomes weaker. In addition, at a high overpotential, the current attenuation of POPy-Py(2) indicated the aggregation of CoTCPP (Figure 3d).  [46]. Reprinted with permission from Ref. [46]. POP is an ideal porous structure, which can be applied for the electrocatalytic redu tion of CO2 through functional design. Dai et al. [48] reported a porous triazine linkin framework TTF-1 with a pyridine linker as the reactive site ( Figure 2a). The existence of well-developed porous structure enables a large contact area between electrodes and ele trolytes, and facilitates the charge and mass transfer (Figure 2b). Amorphous porous po ymer POP often shows a broad pore size distribution, which is attributed to the inte particle void. In particular, a large number of micropores promote the adsorption of CO The permanent CO2 uptake of TTF-1 is up to 64.4 cm 3 g −1 (298 K, 1 bar). TTF-1 also show excellent electrocatalytic activity for CO2 reduction, with a Faraday efficiency of abo 82%.  [47]. Reprinted with permission from Ref. [47]. Copyright 2020 Copyright, Elsevier Ltd. (Amsterdam, The Netherlands).  The size of the pore can affect the charge density of the polymer and the confinemen of the pore to the active site. Tang et al. [49] synthesized a series of porous polymers, POP Py (n) (n means the quantity of benzene rings), through the reaction of benzyl bromide and pyridine. POP-Py (n) with pore sizes from micropore to mesopore were obtained by changing the amount of benzene rings ( Figure 3a). The highly efficient electrocatalyst wa obtained by encapsulating cobalt meso-tetra(4-carboxyphenyl)porphyrin (CoTCPP) (Fig  ure 3b) into POP-Py (n) by the ion exchange method, with single-molecule dispersion POP-Py (0)/CoTCPP showed the highest FEco of 83% at −0.6 V vs. RHE, which is twice a high as the activity of the pure CoTCPP molecular electrocatalytic conversion of CO2 to CO (Figure 3c). The high activity is caused by the monomolecular dispersion of CoTCPP in the pores of POP-Py (0), which promotes the electrochemical accessibility of active sites Moreover, the positively charged substituted pyridine ring in POP-Py can stabilize the key reaction intermediate *COOH to accelerate the conversion of CO2. When a neutra benzyl ring is inserted between two pyridine rings, the positive charge on the pyridine ring will be delocalized through the conjugation effect. Hence, the interaction between POP-Py and key reaction intermediate *COOH becomes weaker. In addition, at a high overpotential, the current attenuation of POPy-Py(2) indicated the aggregation of CoTCPP ( Figure 3d).    [49]. Reprinted with permission from Ref. [49]. Copyright 2020, Royal Society of Chemical (London, UK).
However, many POPs have poor electrical conductivity, which hinders their application in electrochemistry. Alkordi et al. [50] homogenously deposited porous organic polymer containing pyrimidine atop a graphene sheet to provide composite material Py-POP@G ( Figure 4a). It shows significant CO2 electrochemical reduction activity (5 mA cm −2 at −1.6 V), which is much higher than the separated component. PyPOP has an intentional affinity for CO2 due to its rich microporous structure (Figure 4b), while graphene enhances the electrical conductivity of the composite. The synergistic effect of the two components promotes the electrocatalytic reduction of CO2. Later, Alkordi et al. [51] prepared a composite material named POP@MWCNT by synthesizing POP on multi-walled carbon nanotubes in a one-pot method. It was verified by the N2 physical adsorption/desorption test that the high porosity of POP@MWCNT is mainly contributed to by POP, not MWCNTs ( Figure 4c). The Qst of CO2 into the POP and the POP@G are almost the same, which confirms that the adsorption of CO2 by the composites mainly occurs in the pores of POP (Figure 4d). Alkordi et al. proposed post-synthetic modification of the electrode material by initial impregnation in CuBr acetonitrile solution. Impregnation of Cu ions enhances electrode catalytic activity. The current density for the POP-Cu@MWCNTs was enhanced  [49]. Reprinted with permission from Ref. [49]. Copyright 2020, Royal Society of Chemical (London, UK).
However, many POPs have poor electrical conductivity, which hinders their application in electrochemistry. Alkordi et al. [50] homogenously deposited porous organic polymer containing pyrimidine atop a graphene sheet to provide composite material Py-POP@G ( Figure 4a). It shows significant CO 2 electrochemical reduction activity (5 mA cm −2 at −1.6 V), which is much higher than the separated component. PyPOP has an intentional affinity for CO 2 due to its rich microporous structure (Figure 4b), while graphene enhances the electrical conductivity of the composite. The synergistic effect of the two components promotes the electrocatalytic reduction of CO 2 . Later, Alkordi et al. [51] prepared a composite material named POP@MWCNT by synthesizing POP on multi-walled carbon nanotubes in a one-pot method. It was verified by the N 2 physical adsorption/desorption test that the high porosity of POP@MWCNT is mainly contributed to by POP, not MWCNTs ( Figure 4c). The Q st of CO 2 into the POP and the POP@G are almost the same, which confirms that the adsorption of CO 2 by the composites mainly occurs in the pores of POP (Figure 4d). Alkordi et al. proposed post-synthetic modification of the electrode material by initial impregnation in CuBr acetonitrile solution. Impregnation of Cu ions enhances electrode catalytic activity. The current density for the POP-Cu@MWCNTs was enhanced by 75% compared to POP@MWCNTs at −1.5 V (Figure 4e).

Porous Coordination Polymers (PCPs)
Porous coordination polymers (PCPs), also known as metal-organic framework (MOFs), are crystalline materials. Such materials are composed of metal ions or metal par ticles with organic complexes through coordination linkages. PCPs have now become significant category of nanoporous polymers with multiple uses in CO2 storage, CO2 sep aration, and CO2 electrocatalysis due to their organized porous structures with large sur face areas and adjustable pore surface features [52][53][54][55][56]. The catalytic performance of PCP catalysts in CO2 electroreduction is summarized in Table 3. Sun et al. [57] reported a two dimensional bismuth metal-organic framework (Bi-MOF), a catalyst with a microporou structure, for the electrocatalytic reduction of CO2 to HCOOH. They prepared Bi-MOF with tunable grain size and coordination environment by a simple solvothermal method varying the reaction temperature and the type of organic linker. Figure 5d shows the crys tal structure of a Bi-MOF (CAU-17) composed of a triple base linker (1,3,5-benzene tricar boxylic acid, H3BTC). Figure 5a shows a representative type-I N2 adsorption/desorptio isotherm of Bi-MOF, and after measuring it was found that the average pore size wa about 0.8 nm (inset of Figure 5a), indicating a remarkable microporous structure. In par ticular, the obtained Bi-MOF has a CO2 adsorption capacity of 33.0 mgCO2gcat −1 , which i twice the CO2 adsorption capacity of Bi sheets under the same testing conditions (Figur 5b). The superior CO2 adsorption capacity of Bi-MOF is attributed to the large number o differently shaped channels in its structure, which are larger in size than the molecula diameter of CO2. The excellent CO2 uptake capacity promotes the adsorption of larg amounts of CO2 on the surface of Bi-MOF catalysts, thus potentially contributing to highe catalytic conversion. This Bi-MOF shows large Faraday efficiency of HCOOH over a wid potential window, reaching 92.2% at −0.9 V (vs. RHE), and excellent stability over 30 ( Figure 5c).

Porous Coordination Polymers (PCPs)
Porous coordination polymers (PCPs), also known as metal-organic frameworks (MOFs), are crystalline materials. Such materials are composed of metal ions or metal particles with organic complexes through coordination linkages. PCPs have now become a significant category of nanoporous polymers with multiple uses in CO 2 storage, CO 2 separation, and CO 2 electrocatalysis due to their organized porous structures with large surface areas and adjustable pore surface features [52][53][54][55][56]. The catalytic performance of PCPs catalysts in CO 2 electroreduction is summarized in Table 3. Sun et al. [57] reported a twodimensional bismuth metal-organic framework (Bi-MOF), a catalyst with a microporous structure, for the electrocatalytic reduction of CO 2 to HCOOH. They prepared Bi-MOFs with tunable grain size and coordination environment by a simple solvothermal method, varying the reaction temperature and the type of organic linker. Figure 5d shows the crystal structure of a Bi-MOF (CAU-17) composed of a triple base linker (1,3,5-benzene tricarboxylic acid, H 3 BTC). Figure 5a shows a representative type-I N 2 adsorption/desorption isotherm of Bi-MOF, and after measuring it was found that the average pore size was about 0.8 nm (inset of Figure 5a), indicating a remarkable microporous structure. In particular, the obtained Bi-MOF has a CO 2 adsorption capacity of 33.0 mg CO2 g cat −1 , which is twice the CO 2 adsorption capacity of Bi sheets under the same testing conditions (Figure 5b). The superior CO 2 adsorption capacity of Bi-MOF is attributed to the large number of differently shaped channels in its structure, which are larger in size than the molecular diameter of CO 2 . The excellent CO 2 uptake capacity promotes the adsorption of large amounts of CO 2 on the surface of Bi-MOF catalysts, thus potentially contributing to higher catalytic The same microporous structure is a nickel phthalocyanine-based conductive MOF (NiPc-Ni(NH)4) constructed from 2,3,9,10,16,17,23,24-octaaminonickel(II) (NiPc-(NH2)8) as reported by Cao et al. [58]. In this research, Ni-phthalocyanines (NiPc), which was built block-based, with an activated CO2 molecular reduction site, were incorporated into a porous essentially conductive 2D MOF NiPc-Ni(NH)4 for the efficient CO2RR towards the CO2 conversion in an aqueous medium. NiPc-Ni(NH)4 showed a 1.78 nm one-dimensional rectangular channel while running along the c-direction with a distance of 0.34 nm between the stacked two-dimensional conjugated layers in the structural model ( Figure 6a). Therefore, the N atoms of the amino groups in NiPc-(NH2)8 ligands were managed to coordinate with the Ni(II) ion, forming a fully π-conjugated two-dimensional backbone in NiPc-Ni(NH)4. The accessible pore size of ~1.7 nm was obtained by N2 adsorption/desorption isotherms (Figure 6b), and this large microporous structure facilitates the diffusion and mass transfer of reactants to improve the activity of electrocatalytic reactions. Similarly, the porous NiPc-Ni(NH)4 has a great CO2 adsorption capacity (Figure 6c), which indicates that NiPc-Ni(NH)4 with a nitrogen-rich structure has a strong affinity for CO2 to enhance its electrocatalytic activity in CO2RR. It can be seen that NiPc-Ni(NH)4 nanosheets reach a high CO Faraday efficiency of 96.4% at −0.7V (vs. RHE).  [57]. Reprinted with permission from Ref. [57]. Copyright 2020 Copyright Elsevier (Amsterdam, The Netherlands).
The same microporous structure is a nickel phthalocyanine-based conductive MOF (NiPc-Ni(NH) 4 ) constructed from 2,3,9,10,16,17,23,24-octaaminonickel(II) (NiPc-(NH 2 ) 8 ) as reported by Cao et al. [58]. In this research, Ni-phthalocyanines (NiPc), which was built block-based, with an activated CO 2 molecular reduction site, were incorporated into a porous essentially conductive 2D MOF NiPc-Ni(NH) 4 for the efficient CO 2 RR towards the CO 2 conversion in an aqueous medium. NiPc-Ni(NH) 4 showed a 1.78 nm one-dimensional rectangular channel while running along the c-direction with a distance of 0.34 nm between the stacked two-dimensional conjugated layers in the structural model ( Figure 6a). Therefore, the N atoms of the amino groups in NiPc-(NH 2 ) 8 ligands were managed to coordinate with the Ni(II) ion, forming a fully π-conjugated two-dimensional backbone in NiPc-Ni(NH) 4 . The accessible pore size of~1.7 nm was obtained by N 2 adsorption/desorption isotherms (Figure 6b), and this large microporous structure facilitates the diffusion and mass transfer of reactants to improve the activity of electrocatalytic reactions. Similarly, the porous NiPc-Ni(NH) 4 has a great CO 2 adsorption capacity (Figure 6c), which indicates that NiPc-Ni(NH) 4 with a nitrogen-rich structure has a strong affinity for CO 2 to enhance its electrocatalytic activity in CO 2 RR. It can be seen that NiPc-Ni(NH) 4 nanosheets reach a high CO Faraday efficiency of 96.4% at −0.7V (vs. RHE).
Among PCP materials, not only are microporous structures often seen, but also mesoporous structures are common for CO 2 RR catalysts. Gu et al. [59] prepared a series of imidazole (Im)-based zeolite imidazolium salt backbone Ni(Im) 2 nanosheets of different thicknesses using the liquid phase top-down exfoliation method (Figure 7a). Zeolite imidazole frameworks (ZIFs) are a relatively stable class of MOF materials consisting of imidazole groups and metal ions. They stripped layered massive Ni(Im) 2 ZIF to 2D Ni(Im) 2 nanosheets of different thicknesses (5 nm, 15 nm, 65 nm, and 140 nm) and used them as CO 2 RR electrochemical catalysts. As can be seen in Figure 7b, their N 2 adsorption/resolution isotherms are all type VI, which corresponds to a mesoporous structure. It provides a large specific surface area for the catalysts while more reactive sites are exposed, thus ensuring the high catalytic activity of this material. This catalyst was found to achieve 78.8% CO Faraday efficiency at −0.9 V (vs. RHE) with a stability of more than 14 h (Figure 7c). Among PCP materials, not only are microporous structures often seen, but also mesoporous structures are common for CO2RR catalysts. Gu et al. [59] prepared a series of imidazole (Im)-based zeolite imidazolium salt backbone Ni(Im)2 nanosheets of different thicknesses using the liquid phase top-down exfoliation method (Figure 7a). Zeolite imidazole frameworks (ZIFs) are a relatively stable class of MOF materials consisting of imidazole groups and metal ions. They stripped layered massive Ni(Im)2 ZIF to 2D Ni(Im)2 nanosheets of different thicknesses (5 nm, 15 nm, 65 nm, and 140 nm) and used them as CO2RR electrochemical catalysts. As can be seen in Figure 7b, their N2 adsorption/resolution isotherms are all type VI, which corresponds to a mesoporous structure. It provides a large specific surface area for the catalysts while more reactive sites are exposed, thus ensuring the high catalytic activity of this material. This catalyst was found to achieve 78.8% CO Faraday efficiency at −0.9 V (vs. RHE) with a stability of more than 14 h ( Figure  7c). Because a single porous structure is not sufficient for high performance in CO2 capture and electrocatalytic reduction, some researchers have attempted to create hierarchical porous materials through pore engineering strategies [60][61][62][63]. In recent years, the field of MOF has made great progress in the development of hierarchically porous materials [64][65][66][67]. In such hierarchical porous polymers, the inherent micropores of the framework contribute to a high surface area, while the mesopores and/or macropores provide channels for mass transfer and accommodate molecules larger than the micropores [68]. Martin et al. [69] reported the deposition of porous metal-organic skeletons on copper electrodes by electrosynthesis of ionic liquid templates [Cu2(L)] [H4L = 4,4′,4″,4′′′-(1,4-phenylenebis(pyridine-4,2,6-triyl))tetrabenzoic acid]. The structure of Cu2(L) comprises a 3D network built around binuclear [Cu2(OOCR)4] paddlewheels with four bridging carboxylate ligands (Figure 8a,b). The porosity of this MOF was investigated by the N2 adsorption/desorption (c) CO2 adsorption isotherms of NiPc−Ni(NH)4 [58]. Reprinted with permission from Ref. [58]. Copyright 2021 Copyright Springer (Berlin, Germany).
Among PCP materials, not only are microporous structures often seen, but also mesoporous structures are common for CO2RR catalysts. Gu et al. [59] prepared a series of imidazole (Im)-based zeolite imidazolium salt backbone Ni(Im)2 nanosheets of different thicknesses using the liquid phase top-down exfoliation method (Figure 7a). Zeolite imidazole frameworks (ZIFs) are a relatively stable class of MOF materials consisting of imidazole groups and metal ions. They stripped layered massive Ni(Im)2 ZIF to 2D Ni(Im)2 nanosheets of different thicknesses (5 nm, 15 nm, 65 nm, and 140 nm) and used them as CO2RR electrochemical catalysts. As can be seen in Figure 7b, their N2 adsorption/resolution isotherms are all type VI, which corresponds to a mesoporous structure. It provides a large specific surface area for the catalysts while more reactive sites are exposed, thus ensuring the high catalytic activity of this material. This catalyst was found to achieve 78.8% CO Faraday efficiency at −0.9 V (vs. RHE) with a stability of more than 14 h ( Figure  7c). Because a single porous structure is not sufficient for high performance in CO2 capture and electrocatalytic reduction, some researchers have attempted to create hierarchical porous materials through pore engineering strategies [60][61][62][63]. In recent years, the field of MOF has made great progress in the development of hierarchically porous materials [64][65][66][67]. In such hierarchical porous polymers, the inherent micropores of the framework contribute to a high surface area, while the mesopores and/or macropores provide channels for mass transfer and accommodate molecules larger than the micropores [68]. Martin et al. [69] reported the deposition of porous metal-organic skeletons on copper electrodes by electrosynthesis of ionic liquid templates [Cu2(L)] [H4L = 4,4′,4″,4′′′-(1,4-phenylenebis(pyridine-4,2,6-triyl))tetrabenzoic acid]. The structure of Cu2(L) comprises a 3D network built around binuclear [Cu2(OOCR)4] paddlewheels with four bridging carboxylate ligands (Figure 8a,b). The porosity of this MOF was investigated by the N2 adsorption/desorption Figure 7.
Because a single porous structure is not sufficient for high performance in CO 2 capture and electrocatalytic reduction, some researchers have attempted to create hierarchical porous materials through pore engineering strategies [60][61][62][63]. In recent years, the field of MOF has made great progress in the development of hierarchically porous materials [64][65][66][67]. In such hierarchical porous polymers, the inherent micropores of the framework contribute to a high surface area, while the mesopores and/or macropores provide channels for mass transfer and accommodate molecules larger than the micropores [68]. Martin et al. [69] reported the deposition of porous metal-organic skeletons on copper electrodes by electrosynthesis of ionic liquid templates [Cu 2 (L)] [H4L = 4,4 ,4 ,4 -(1,4phenylenebis(pyridine-4,2,6-triyl))tetrabenzoic acid]. The structure of Cu 2 (L) comprises a 3D network built around binuclear [Cu 2 (OOCR) 4 ] paddlewheels with four bridging carboxylate ligands (Figure 8a,b). The porosity of this MOF was investigated by the N 2 adsorption/desorption test at 77 K, and Cu2(L)-e exhibited a distribution between type I and type IV, indicating the presence of both mesopores and micropores. The distribution of micropores and mesopores in both materials was analyzed by Horvath Kawazoe and Barrett-Joyner-Halenda (BJH) methods, respectively (Figure 8c,d). By contrast, the reduction of micropores in Cu 2 (L)-e resulted in lower N 2 uptake at low pressure, while the total pore volume of Cu 2 (R)-e produced by the template effect of the ionic liquid during electrolytic synthesis was 1.89 cm 3 g −1 , significantly larger than that of Cu 2 (L)-t (0.32 cm 3 g −1 ), reflecting the presence of Cu 2 (L)-e-mediated pores. The presence of mesopores also facilitates the mass transfer process during the reaction. As a result, this electrode showed excellent activity for the electroreduction of CO 2 to formic acid with a low starting potential of −1.45 V (vs. Ag/AgCl) and HCOOH Faraday efficiency reaching 90.5% at −1.80 V (vs. Ag/AgCl). Despite a similar micro-and mesoporous structure, the role of pore structure in the materials reported by Lan et al. [70] is quite different. The specific surface area of the material decreases instead after the introduction of PPy in the material, which is due to the encapsulation of PPy in the pores of MOF-545-Co. The formation of PPy occurs in the MOF channel, and after the formation of PPy, there will be a π-π interaction between the PPy molecules and TCPP molecules in the MOF structure similar to the host-object interaction, and this interaction will facilitate the charge transfer process during the reaction. Meanwhile, the results of CO 2 adsorption measurements showed that the addition of PPy was quite beneficial for CO 2 adsorption. As a result, the CO 2 reduction efficiency is significantly enhanced.
(BJH) methods, respectively (Figure 8c,d). By contrast, the reduction of micropores in Cu2(L)-e resulted in lower N2 uptake at low pressure, while the total pore volume of Cu2(R)-e produced by the template effect of the ionic liquid during electrolytic synthesis was 1.89 cm 3 g −1 , significantly larger than that of Cu2(L)-t (0.32 cm 3 g −1 ), reflecting the presence of Cu2(L)-e-mediated pores. The presence of mesopores also facilitates the mass transfer process during the reaction. As a result, this electrode showed excellent activity for the electroreduction of CO2 to formic acid with a low starting potential of −1.45 V (vs. Ag/AgCl) and HCOOH Faraday efficiency reaching 90.5% at −1.80 V (vs. Ag/AgCl). Despite a similar micro-and mesoporous structure, the role of pore structure in the materials reported by Lan et al. [70] is quite different. The specific surface area of the material decreases instead after the introduction of PPy in the material, which is due to the encapsulation of PPy in the pores of MOF-545-Co. The formation of PPy occurs in the MOF channel, and after the formation of PPy, there will be a π-π interaction between the PPy molecules and TCPP molecules in the MOF structure similar to the host-object interaction, and this interaction will facilitate the charge transfer process during the reaction. Meanwhile, the results of CO2 adsorption measurements showed that the addition of PPy was quite beneficial for CO2 adsorption. As a result, the CO2 reduction efficiency is significantly enhanced.  [69]. Reprinted with permission from Ref. [69]. Copyright 2020 Copyright Nature (Oxford, England).

Covalent Organic Frameworks (COFs)
Covalent organic frameworks (COFs) are a class of reticulated porous polymeric materials in which the constructed organic blocks are connected by covalent bonds. COFs are characterized by their tunable porosity, which gives them a great accessible surface area and an abundance of active sites. It is due to this property that they have also been extensively studied in catalytic and energy storage systems [80,81]. In recent years, COFs have been widely recognized by researchers as an excellent material that can be used for electrocatalytic CO 2 reduction reactions due to their rich pore structure that can facilitate the adsorption and desorption of gas molecules during the reaction. By consciously introducing functional units such as bipyridyl, porphyrin, and phthalocyanine in the COF backbone to link metal ions for electrocatalytic CO 2 reduction reactions, they have been widely explored in CO 2 reduction reactions [82][83][84].
Recently, fluorinated COFs have also been effective catalysts for electrocatalysis. CTFs are a class of COF materials consisting of three elements, C, N, and H, and aromatic triazine units connected by covalent bonds. Since CTFs usually have a large specific surface area, high nitrogen content, excellent stability, and electrical conductivity, CTFs have a wide range of applications in gas separation and storage, energy storage, and electrocatalysis [85][86][87][88][89][90]. We summarize the catalytic performance of COF catalysts in CO 2 electroreduction in Table 4. Dai et al. [91] synthesized a fluorinated covalent triazine skeleton (F-CTF-1) with the low-temperature ionothermal method using 2,3,5,6tetrafluoroterephthalonitrile as a monomer (Figure 9a,b). The BET specific surface area of this fluorinated COF-containing material was 367 m 2 g −1 and mainly microporous (Figure 9c-e). The introduction of fluorine enhanced the hydrophobic and pro-CO 2 properties of F-CTF-1 and improved the interaction between F-CTF-1 and CO 2 molecules. As an outcome, F-CTF-1 exhibited great Faraday efficiency (95.7%) at −0.8 V (vs. RHE) and was used for the electroreduction of CO 2 to CO. Also with a microporous structure, Yaghi and colleagues [92] synthesized a 2D COF (COF-366-Co) in which the cobalt porphyrin site could catalyze the conversion of CO 2 to CO by CO 2 RR. They synthesized a model framework (COF-366-Co) by the imine condensation of 5,10,15,20-tetrakis[(4aminophenyl)porphinato]cobalt[Co(TAP)] with 1,4-benzenedicarboxaldehyde (BDA). The porous COF material was evacuated by activation with supercritical carbon dioxide and heating to 100 • C for 18 h. The surface area was determined to be 1360 m 2 g −1 . The adsorption branch fitted using density functional theory (DFT) showed a relatively narrow pore size distribution (10 to 18 Å), consistent with the proposed model. When tested in an aqueous solution, COF-336-Co showed a high Faraday efficiency of 90% Co (FE CO ), which is 10% higher than that of the molecular cobalt porphyrin catalyst. It is therefore inferred that micropores would allow for a higher CO 2 adsorption capacity in the framework, and the greater electrocatalytic activity and chemical availability of the catalytic cobalt porphyrin active site.
It was found that the difference in pore size in similar structures also affects the performance of COF catalysts. Recently, Huang et al. [93] constructed two catalysts, CuPcF 8 -CoNPc-COF and CuPcF 8 -CoPc-COF (Figure 10a), with similar pore structures. The adsorption curves of both COFs were classified as typical type I isotherms, which is characteristic of micropores (Figure 10c,d). The surface areas of CuPcF 8 -CoPc-COF and CuPcF 8 -CoNPc-COF were determined to be 376 and 452 m 2 g −1 , respectively. Correspondingly, their pore volumes were calculated to be 0.23 and 0.30 cm 3 g −1 . Non-local density flooding theory (NLDFT) was used to evaluate their pore sizes of 1.0 and 1.2 nm (Figure 6c,d). Meanwhile, the CO 2 capacities of CuPcF 8 -CoPc-COF and CuPcF 8 -CoNPc-COF were 18 and 35 mg g −1 , respectively. It can be seen that CuPcF 8 -CoNPc-COF has a larger pore size, specific surface area, and better CO 2 capacity compared to CuPcF 8 -CoPc-COF, which is more favorable to improve the catalytic activity of CO 2 RR. Thus, when tested in 0.5 M CsHCO 3 , the FE CO of CuPcF 8 -CoPc-COF reached a peak of 91% at −0.70 V (vs. RHE), while the FE CO of CuPcF 8 -CoNPc-COF was as high as 97% at −0.62 V (vs. RHE). the molecular cobalt porphyrin catalyst. It is therefore inferred that micropores would allow for a higher CO2 adsorption capacity in the framework, and the greater electrocatalytic activity and chemical availability of the catalytic cobalt porphyrin active site. It was found that the difference in pore size in similar structures also affects the performance of COF catalysts. Recently, Huang et al. [93] constructed two catalysts, CuPcF8-CoNPc-COF and CuPcF8-CoPc-COF (Figure 10a), with similar pore structures. The adsorption curves of both COFs were classified as typical type I isotherms, which is characteristic of micropores (Figure 10c,d). The surface areas of CuPcF8-CoPc-COF and CuPcF8-CoNPc-COF were determined to be 376 and 452 m 2 g −1 , respectively. Correspondingly, their pore volumes were calculated to be 0.23 and 0.30 cm 3 g −1 . Non-local density flooding theory (NLDFT) was used to evaluate their pore sizes of 1.0 and 1.2 nm (Figure 6c,d). Meanwhile, the CO2 capacities of CuPcF8-CoPc-COF and CuPcF8-CoNPc-COF were 18 and 35 mg g −1 , respectively. It can be seen that CuPcF8-CoNPc-COF has a larger pore size, specific surface area, and better CO2 capacity compared to CuPcF8-CoPc-COF, which is more favorable to improve the catalytic activity of CO2RR. Thus, when tested in 0.5 M CsHCO3, the FECO of CuPcF8-CoPc-COF reached a peak of 91% at −0.70 V (vs. RHE), while the FECO of CuPcF8-CoNPc-COF was as high as 97% at −0.62 V (vs. RHE). Similarly, mesoporous structures are also present in COF materials, and Zeng et al. [94] formed COF on the surface of Mg/Al-LDH, and the obtained catalysts showed a layered and porous morphology with a thickness of about 36 nm and an abundance of active sites. The N2 adsorption isotherm at 77 K showed a BET surface area of 123 m 2 g −1 for 2D-Co-COF500 (Figure 11a). The pore size distribution plot shows that the pore volume of 2D-CO-COF500 is 0.52 cm 3 g −1 , where most of the pores are between 25 and 80 nm in diameter (Figure 11b). Electrochemical tests were then performed on this material. The FE of CO was found to be 66.1% at −0.5 V, 84.0% at −0.6 V, 92.1% at −0.7 V, 96.5% at −0.8 V, 96.0 at −0.9 V, and finally 80.2% at −1.0 V (Figure 11c), while electrochemical tests on CO- Figure 10.
Similarly, mesoporous structures are also present in COF materials, and Zeng et al. [94] formed COF on the surface of Mg/Al-LDH, and the obtained catalysts showed a layered and porous morphology with a thickness of about 36 nm and an abundance of active sites. The N 2 adsorption isotherm at 77 K showed a BET surface area of 123 m 2 g −1 for 2D-Co-COF500 (Figure 11a). The pore size distribution plot shows that the pore volume of 2D-CO-COF500 is 0.52 cm 3 g −1 , where most of the pores are between 25 and 80 nm in diameter (Figure 11b). Electrochemical tests were then performed on this material. The FE of CO was found to be 66.1% at −0.5 V, 84.0% at −0.6 V, 92.1% at −0.7 V, 96.5% at −0.8 V, 96.0 at −0.9 V, and finally 80.2% at −1.0 V (Figure 11c), while electrochemical tests on CO-COF500 revealed that the FEs of CO were all at the same test potential. Among them, 2D-Co-COF500 reached a maximum FE of 96.5% for CO at −0.8 V, and the catalytic performance was better than most of the current COF materials. To further investigate the catalyst activity, the CO current density (j CO ) was tested. It was found that 2D-CO-COF500 had the highest current density of 17.9 mA cm −2 at a potential of −1.0 V (Figure 11d), while Co-COF500 also reached a maximum current density of 6.8 mA cm −2 at −0.9 V. For materials with multi-level pore structures with both micro-and mesopores, the large specific surface area and rich pore structure allows them to have a strong CO2 adsorption capacity, enhanced mass transfer, and more exposed reactive sites, ultimately leading to stronger CO2 reduction performance. Zhang et al. [95] used porphyrin-based covalent organic framework (Por-COF) nanosheets vertically anchored to carbon nanotubes (CNT) by covalent linkage for an efficient electrocatalytic CO2 reduction reaction. The porosity structure of Por-COF and MWCNT-Por-COF-M were investigated by the BET test at 77 K. The results indicate that all the samples exhibit type I adsorption isotherms and the pore size distributions are concentrated on 15 Å (Figure 12a,b). The electrochemical test results show that the MWCNT-Por-COF-M (M: Co, Ni, Fe) material has higher electrocatalytic performance and Faraday efficiency than Por-COF-M. Specifically, MWCNT-Por-COF-Co achieves the highest Faraday efficiency of 99.3% for CO at -0.6 V, and the highest current density of 18.77 mA cm −2 at −1.0 V and TOF of 70.6 s −1 (Figure 12c). The electrochemical test results indicate that the excellent electrocatalytic activity of MWCNT-Por-COF-Co is due to the effective electron transfer. In addition, they used MWCNT-Por-COF-Cu in a flow cell for a CO2RR test with electrolytes in 1.0 M KOH. The test results illustrate that MWCNT-Por-COF-Cu has a higher CO2 catalytic efficiency than MWCNT-Por-COF, the product is mainly CH4, and the Faraday efficiency can reach up to 71.2% (Figure 12d). Sun et al. [96] used tetraanhydrides of 2,3,9,10,16,17,23,24-octacarbox- For materials with multi-level pore structures with both micro-and mesopores, the large specific surface area and rich pore structure allows them to have a strong CO 2 adsorption capacity, enhanced mass transfer, and more exposed reactive sites, ultimately leading to stronger CO 2 reduction performance. Zhang et al. [95] used porphyrin-based covalent organic framework (Por-COF) nanosheets vertically anchored to carbon nanotubes (CNT) by covalent linkage for an efficient electrocatalytic CO 2 reduction reaction. The porosity structure of Por-COF and MWCNT-Por-COF-M were investigated by the BET test at 77 K.
The results indicate that all the samples exhibit type I adsorption isotherms and the pore size distributions are concentrated on 15 Å (Figure 12a,b). The electrochemical test results show that the MWCNT-Por-COF-M (M: Co, Ni, Fe) material has higher electrocatalytic performance and Faraday efficiency than Por-COF-M. Specifically, MWCNT-Por-COF-Co achieves the highest Faraday efficiency of 99.3% for CO at -0.6 V, and the highest current density of 18.77 mA cm −2 at −1.0 V and TOF of 70.6 s −1 (Figure 12c). The electrochemical test results indicate that the excellent electrocatalytic activity of MWCNT-Por-COF-Co is due to the effective electron transfer. In addition, they used MWCNT-Por-COF-Cu in a flow cell for a CO 2 RR test with electrolytes in 1.0 M KOH. The test results illustrate that MWCNT-Por-COF-Cu has a higher CO 2 catalytic efficiency than MWCNT-Por-COF, the product is mainly CH 4 , and the Faraday efficiency can reach up to 71.2% (Figure 12d). Sun et al. [96] used tetraanhydrides of 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine cobalt(II) (CoTAPc) as a junction to couple with 5,15-di(4-aminophenyl)-10,20-diphenylporphyrin (DAPor) or 5,15,10,20-tetrayl(4-aminophenyl)porphyrin (TAPor) by imination reaction to fabricate novel coupled phthalocyanine-porphyrin type 1:2 (CoPc-2H 2 Por) or type 1:1 (CoPc-H 2 Por) COFs. Both type 1:2 and type 1:1 COFs were found to exhibit over 90% maximum Faraday efficiency and high stability in CO 2 electrocatalytic reduction reactions. Meanwhile, CoPc-2H 2 Por has a larger pore size and a more conjugated structure than CoPc-H 2 Por, leading to more efficiency in electron transfer, more adsorption and reaction of CO 2 , and rapid proton transfer during the reaction, which ultimately leads to higher reaction kinetics, resulting in better CO 2 RR activity than CoPc-H2Por. more adsorption and reaction of CO2, and rapid proton transfer during the reaction, which ultimately leads to higher reaction kinetics, resulting in better CO2RR activity than CoPc-H2Por.

Nitrogen-Containing Polymers (N-Polymers)
There is also a class of porous polymeric materials called "metal-loaded ligand-active nitrogen-containing polymers" (N-polymers). This kind of N-polymer is characterized by the presence of nitrogen functional groups that complex metal cations or anchor metal nanoparticles within the polymer backbone (e.g., polyaniline and polypyrrole) or side chains (e.g., polyvinyl terbium pyridine and poly (4-vinyl pyridine)). The application of such materials in the field of CO 2 electroreduction, however, dates back to three decades ago, when Wrighton's [106] and Meyer's groups [107] introduced N-polymers in the field of non-homogeneous CO 2 electroreduction and found that such catalysts have excellent catalytic properties in CO 2 conversion. So far, metal-doped N-polymer materials have been tested to synthesize polycarbon products up to propanol. CO 2 adsorption by porous polymers has been used as a competitive strategy for selective CO 2 removal/capture [108][109][110]. Due to the rapid development of nitrogencontaining nanoporous polymeric materials, the application of N-polymers in CO 2 capture has achieved remarkable success. Nitrogen-rich carbons are of great interest because of the contribution of N as a basic or polar site for effective interaction with CO 2 [111,112]. Therefore, the preparation of nitrogen-rich carbon materials for CO 2 capture is quite attractive and important in various applications such as CO 2 capture. Due to the high porosity and relatively high nitrogen content, the adsorption of polyaniline (PANI) is very effective in both liquid and gas phases [113][114][115]. Park and his colleagues [116] prepared a porous carbon material using melamine and PANI with milder reaction conditions (≤650 • C). The prepared materials (MPCs) were found to have some CO 2 adsorption capacity and therefore could be used for CO 2 adsorption. It can be seen from Figure 13a that the adsorption capacity of PDCs for N 2 increases with the increase in the pyrolysis temperature. Therefore, for N 2 adsorption capacity, MPC-500 is the worst while MPC-650 is the best. The pore size distribution (PSD) of the material was also obtained from the isothermal curve shown in Figure 13a and shown in Figure 13b by nonlocal linear density functional theory (NLDFT). The adsorption capacities of the different materials prepared for CO 2 and N 2 are depicted from Figure 13c. As shown in Figure 13a, the adsorption of CO 2 decreases according to the decrease in the pyrolysis temperature of the material. Thus, MPC-600 has the highest CO 2 adsorption capacity and MPC-500 has the lowest CO 2 adsorption capacity. Among them, MPC-550 showed superior CO 2 selectivity, attributing this performance to the increase in the elemental N content of the material, the decrease in porosity, or the decrease in pore size and the increase in defects.
(NLDFT). The adsorption capacities of the different materials prepared for CO2 and N2 are depicted from Figure 13c. As shown in Figure 13a, the adsorption of CO2 decreases according to the decrease in the pyrolysis temperature of the material. Thus, MPC-600 has the highest CO2 adsorption capacity and MPC-500 has the lowest CO2 adsorption capacity. Among them, MPC-550 showed superior CO2 selectivity, attributing this performance to the increase in the elemental N content of the material, the decrease in porosity, or the decrease in pore size and the increase in defects.  P4VP is an N-heteroaromatic polymer with a mesoporous structure that allows easy diffusion of small molecules and anions through the polymer network [117,118]. Nowadays, this N-polymer has been introduced into the electrochemical reduction of CO 2 reactions, and Koper and colleagues [119] showed a significant improvement in the selectivity of polycrystalline Cu and Au electrodes for HCOOH at low overpotentials after chemical modification by using poly(4-vinyl pyridine) (P4VP) layers. Measurements showed that the hydrophobicity of the P4VP layer limited the mass transport of HCO 3 − and H 2 O, but had little effect on the mass transport of CO 2 due to its mesoporous structure.
Meanwhile, g-C 3 N 4 is also a common N-polymer material with a structure in which the C and N atoms are sp2-hybridized to form a π-type conjugated system with a highly delocalized domain. Due to its high stability no matter the extreme chemical environment or high temperature, proper pore structure, redox potential, and abundant functional groups on the surface, there are also many studies based on g-C 3 N 4 materials reported for energy and environmental applications [120,121], especially for electrocatalytic CO 2 reduction [122]. Zhao et al. [123] prepared a Au-CDots-C 3 N 4 material for CO 2 RR that can efficiently convert CO 2 to CO. Electrochemical tests showed that 4 wt% Au-CDots-C 3 N 4 has optimal CO 2 electrocatalytic performance, and this material can undergo CO 2 electrocatalytic reactions at −0.5 V and the Faraday efficiency of CO reached a maximum of 79.8%. The surface area of 4 wt% Au-CDots-C 3 N 4 was investigated by the BET method at 77K. Figure 14a demonstrates the BET plot of 4 wt% Au-CDots-C 3 N 4 , which is consistent with a typical type IV isotherm with an H3-type hysteresis loop, indicating a large number of mesopores in this material. The specific surface area of this material was tested to be 117 m 2 g −1 , which originates from the rich void structure unique to the C 3 N 4 matrix. Additionally, the pore size distribution obtained by the BJH method test can be shown by Figure 14a. Hu et al. [124] tried to modify Cu and Ru onto g-C 3 N 4 (Cu x Ru y CN) and use it as an electrode for electrocatalytic CO 2 reduction, and it can be seen from the test results that Cu and Ru modification on g-C 3 N 4 can be an effective way to improve its electrocatalytic performance for energy applications. As can be seen in Figure 14a, the surface area of the g-C 3 N 4 sample decreases with increasing Cu content, which indicates that the increased Ru and tiny CuO particles partially block the mesopores. Therefore, this result indicates that the Cu x Ru y CN samples with proper Cu and Ru ratios show the best BET surface area, pore volume and pore size under optimized Cu and Ru decoration to attract reactant molecules and provide active sites for better electrocatalytic reactions.
To summarize, the influence of the structural characteristics of four different types of porous polymer materials, POPs, PCPs, COFs, and N-polymers, on CO 2 capture and electrocatalytic reduction have been reviewed. Their structural features and advantages and disadvantages for CO 2 capture and electrocatalytic reduction are further summarized in Table 5.
Cu and Ru modification on g-C3N4 can be an effective way to improve its electrocatalytic performance for energy applications. As can be seen in Figure 14a, the surface area of the g-C3N4 sample decreases with increasing Cu content, which indicates that the increased Ru and tiny CuO particles partially block the mesopores. Therefore, this result indicates that the CuxRuyCN samples with proper Cu and Ru ratios show the best BET surface area, pore volume and pore size under optimized Cu and Ru decoration to attract reactant molecules and provide active sites for better electrocatalytic reactions.  [124]. Reprinted with permission from Ref. [124].
To summarize, the influence of the structural characteristics of four different types of porous polymer materials, POPs, PCPs, COFs, and N-polymers, on CO2 capture and electrocatalytic reduction have been reviewed. Their structural features and advantages and Figure 14. (a) BET plot of 4 wt% Au−CDots−C 3 N 4 ; (b) BET surface area (black), pore diameter (green), and pore volume (red) of the Cu x Ru y CN [124]. Reprinted with permission from Ref. [124]. Copyright 2021 Copyright Elsevier (Amsterdam, The Netherlands). Table 5. Advantages and disadvantages of porous polymer materials for CO 2 capture and electrocatalytic reduction.

Materials Structural Features Advantages Disadvantages
noting that the introduction of heteroatoms can effectively improve the affinity of porous polymers to CO 2 compared with other gases. Heteroatoms act as binding sites of CO 2 through dipole-quadrupole interaction. In addition, the functionalization of the surface CO 2 -philic groups helps to further improve the adsorption capacity and selectivity. The role of pore structures in CO 2 adsorption and electroreduction should be particularly emphasized. An abundant variety of building blocks with various functional groups are utilized for porous materials' synthesis, which allows the pore structure and the chemical environment within the pore to be designed for a specific purpose. The microporous and ultramicroporous structures are considered to be preferred for the adsorption of CO 2 molecules. The physical and chemical interactions between pore walls and CO 2 molecules can be enhanced by fine-tuning the pore size and structure, which improves the catalytic efficiency of the electrocatalyst. The mesoporous structure is conducive to the increase in the accessibility of catalytic reaction sites, mass transfer, and the enrichment of reactants in the pores, which will accelerate the rate of the CO 2 electrocatalytic reduction reaction. The hierarchical porous structure generally consists of a composite of micropores and mesopores. Although the surface area and pore volume of the polymers decrease as the mesopores increase, the disadvantage of single micropores that are not conducive to the diffusion of reactants and products at a single microporous pore size is overcome. Hierarchical porous polymers are promising candidates for expanding the electrocatalyst family. Effective electrocatalytic reactions occur at certain active sites. Therefore, it is of practical significance to create and regulate porous structures in order to expose more catalytic active sites.
Despite the above progresses achieved, there are still some great challenges to overcome in the design and synthesis of porous polymers towards highly efficient CO 2 capture and catalytic reduction. Promising future research directions in this field are also prospected as follows.
Firstly, the specific surface area, pore size, and pore morphology of porous polymer materials are tuned by functionalized modification of the polymer backbone to achieve the result of improved catalytic performance, for example, functional elements of a semiconductor nature can be introduced into the backbone structure, thus enabling porous polymer materials to achieve and improve performance in electrocatalysis, energy storage, etc. However, for now, achieving specific tuning of porous polymer materials is still a great challenge. More attention should be paid to boosting the coordination of active sites and pore topology to prevent the separation of metal ions or mixtures while improving the electrocatalytic performance, so as to provide long-term stability. Hierarchical porous structures can be constructed by developing advanced synthetic methods to improve the adsorption and activation of electrocatalysts for CO 2 . The chemical environment within the pores can be regulated by introducing heteroatoms, substituents, and metal components to stabilize key reaction intermediates and promote the charge transfer properties of porous polymers.
Secondly, the mechanism of porous polymers in the electrocatalytic reduction of CO 2 is not well understood, especially for the exploration of the pathway of polycarbonate product generation. In the current study, different reaction intermediates are detected corresponding to different reaction mechanisms, but most of them stay in proposing the mechanism, and the investigation of experimental evidence is not comprehensive. It is difficult to accurately predict the electrocatalytic performance of porous polymers only by theoretical calculation, which limits the development of novel advanced porous polymers for CO 2 electrocatalysts. Development of cutting-edge in situ monitoring technologies could bring about promising solutions to this issue. In situ surface characterization techniques, such as XPS, TEM, XRD, and XAS, can provide clear information about the evolution of the active sites of the electrocatalyst and the intermediate species of key reactions, which will deepen the understanding of CO 2 capture and catalytic reduction by porous polymers.
Third, the lab-scale research generally utilizes concentrated CO 2 for testing, which is quite different from the atmospheric CO 2 in practical situations. To facilitate the conversion of basic research achievements into products in industry, atmospheric samples are recommended to be used in basic research. In addition, most of the existing synthetic means are costly and have harsh reaction conditions, making it difficult to achieve low-cost mass production. We should focus on finding environmentally friendly, simple, and inexpensive co-production methods to promote the industrial application of porous polymer materials.
In conclusion, porous polymers are highly efficient CO 2 capture and conversion materials with great development potential. There are still severe challenges including lack of accurate design of porous polymers with well-coordinated catalytic active sites and pore topology, insufficient cutting-edge in situ monitoring technologies, as well as impractical testing conditions and high production costs. Through reasonable structure design, in-depth research on electrocatalysis mechanisms, and reduction of production cost, the practicability of porous polymers can be further promoted.  Data Availability Statement: Data sharing is not applicable to this article as no new data were created in this study.

Conflicts of Interest:
The authors declare no conflict of interest.